This invention relates to high strength aluminum products, and particularly to methods for increasing the toughness of such products without substantial loss of strength.
High strength aluminum alloys and composites are required in certain applications, notably the aircraft industry where the combination of high strength, high stiffness and low density is particularly important. High strength is generally achieved in aluminum alloys by combinations of copper, zinc and magnesium, and high stiffness is generally achieved by metal matrix composites such as those formed by the addition of silicon carbide, boron carbide or aluminum oxide particles to an aluminum matrix. Recently, aluminum-lithium alloys containing 2.0-2.8% lithium by weight have been developed. These alloys possess a lower density and higher elastic modulus than conventional non-lithium-containing alloys.
The preparation and properties of aluminum-based alloys containing lithium are widely disclosed, notably in J. Stone & Company, British Pat. No. 787,665 (Dec. 11, 1957); Ger. Offen. No. 2,305,248 (National Research Institute for Metals, Tokyo, Jan. 24, 1974); Raclot, U.S. Pat. No. 3,343,948 (Sept. 26, 1967); and Peel et al., British Pat. No. 2,115,836 (Sept. 14, 1983). Powder metallury techniques involving the blending of powdered constituents have been disclosed for a variety of purposes, notably by Fujitsu, Ltd., Japanese Pat. No. 53-75107 (1976); Giorgi et al., U.S. Pat. No. 3,713,898 (Jan. 30, 1973); and Reen, U.S. Pat. No. 3,713,817 (Jan. 30, 1973).
It is also well known that alloys can be made by mixing elemental powders and heating the mixture to a temperature high enough to cause diffusion to take place and form an alloy of uniform composition. See The Physics of Powder Metallurgy, W. E. Kingston, ed., p. 372, McGraw Hill, New York (1951); and C. G. Goetzel, Treatise on Powder Metallurgy, vol. 11, p. 492, Interscience Publishers Inc., New York (1950). Because of the difficulties inherent in obtaining homogeneity, however, the usual practice in aluminum and other alloy systems is to form an alloy powder directly from a prealloyed melt.
Unfortunately, high strength aluminum materials are frequently characterized by low toughness, as evidenced by impact tests on notched specimens (e.g., Charpy tests) and by fracture toughness tests on fatigue precracked specimens where the critical stress intensity factors are determined.